1. Field of Invention
The invention relates to a method of manufacturing a liquid crystal display (LCD) panel and, in particular, to a method of manufacturing a thin film transistor matrix substrate.
2. Related Art
In recent years, opto-electronic related technologies have made tremendous progress. The digital era promotes the development of the LCD market. Since they have such advantages as high picture quality, small size, light weight, low driving voltages, and low power consumption, the LCD's have been widely used in personal digital assistants (PDA), mobile phones, camcorders, notebook computers, desktop displays, vehicle displays, and projection televisions. They have gradually replaced the traditional cathode ray tubes (CRT) as the mainstream of displays.
The LCD is a display device that utilizes liquid crystals to achieve display effects. Since it is better than the traditional CRT in size and weight, it is often used in various kinds of personal systems, from as small as mobile phones, PDA's, displays on digital cameras to as large as televisions and advertisement boards.
The reason that the LCD is better than the traditional CRT in size and weight is because most of the devices in the LCD are flat. After cutting these devices to appropriate sizes, it is thus possible to have light and flat LCD's.
It is well-known that photo masks are expensive. More photo masks mean a higher cost as well as longer manufacturing time. In addition to reducing the cost, reducing the number of photo masks also speed up the production and competitive power of the products. Therefore, the invention aims at reducing the number of photo masks required for making amorphous silicon LCD's and thus reducing the production cost and enhancing the competitive power of the products.
If transistors are processed using the inverted staggered back-channel protection method in the usual process of making thin film transistor matrix substrates, then the transistor channels are not damaged by etching and the devices keep their good properties. Also due to the inverted staggered back-channel protection, the film thickness of the semiconductor layer is reduced. This does not decrease the production of optical current, but also reduces the plasma damages during the film formation. Therefore, using the inverted staggered back-channel protection method to make transistor matrix substrates is the common practice in the field.
In the usual process of manufacturing thin film transistor matrix substrates, particularly the inverted staggered back-channel protection method, the number of required photo masks is reduced down to five or four. Take the four-photo-mask process as an example. The key steps of reducing the number of photo masks are shown in FIG. 1. A gate 102 is formed on the glass substrate 100, followed by the deposition of a dielectric layer 104, a semiconductor layer 106, an ohmic contact layer 108, and an electrode layer 110 in sequence. After coating a photo resist (PR) layer (not shown), a halftone photo mask is used to perform exposure and subsequent developing steps, forming a patternized PR layer 112. The patternized PR layer 112 is formed over the gate 102 in a concave shape in the region for forming source/drain and channels. Transistors are then formed using different etching methods. The PR layer 112 has a smaller thickness h1 at the location for the channel, while other part of the PR layer 112 has a thickness h2. The thickness h1 of the PR layer 112 has two different heights in the region for forming the source/drain and channel. This is the key to the entire process. Since each region has to have two different PR heights, both the taper angle and the PR thickness h1 on the channel region will affect subsequent etching. However, it is very difficult to control these two factors.
Therefore, how to reduce the number of required photo masks while retaining the yield is a problem.